1. Field of the Invention
The present invention relates to a sheet conveying apparatus conveying two sheets in an overlapped state and an image forming apparatus having the same.
2. Description of the Related Art
In an existing sheet post-processing device with a post-processing unit such as a stapler, a subsequent sheet may not be received during the execution of sheet post-processing of a precedent sheet since it takes time for the sheet post-processing such as aligning and stapling. In order to solve this problem, there is known a configuration in which the sheet post-processing time of the precedent sheet is ensured by conveying sheets while a predetermined number of subsequent sheets conveyed during the sheet post-processing of the precedent sheet are overlapped on each other.
As an apparatus that conveys sheets in an overlapped state, there is known a conveying apparatus disclosed in, for example, Japanese Patent Laid-Open No. 10-152253. The sheet conveying apparatus includes two pairs of rollers having different circumferential velocities. Here, when a sheet is nipped between a large-diameter roller having a slow circumferential velocity and a pinching roller, the leading end of the subsequent sheet sent from the roller having a fast circumferential velocity enters between the precedent sheet and the large-diameter roller.
More specifically, the apparatus has the following configuration. A sheet which is conveyed fast by a conveying roller decreases in speed to an intermediate speed in a speed decreasing region existing at the downstream region in relation to a position facing the conveying roller. When the sheet is further conveyed in an intermediate conveying portion, the leading end of the sheet contacts the large-diameter roller having a slow circumferential velocity so that the leading end is drawn thereto. The subsequent sheet also decreases in speed in this way and catches up the precedent sheet. At this time, the precedent sheet is nipped between the large-diameter roller and the pinching roller and largely turns along the peripheral surface of the large-diameter roller, so that the tail end separates from the roller and jumps along the outer shape of the large-diameter roller. For this reason, the leading end of the subsequent sheet enters between the tail end of the precedent sheet and the large-diameter roller, so that the sheets may be conveyed in a overlapped state.
However, in the invention disclosed in Japanese Patent Laid-Open No. 10-152253, the shift amount of two sheets may not be stabilized when the gap between the sheets changes due to a variation in the sheet feeding timing because of the configuration in which the sheets are overlapped on each other while being shifted in the conveying direction using the speed reduction mechanism. When there is a reference wall which is provided at the downstream and serves as a reference for aligning the sheets in the conveying direction upon conveying two overlapped sheets, two sheets are overlapped on each other in a shifted state so that the precedent sheet advances in the conveying direction in relation to the subsequent sheet and hence the downstream end in the conveying direction of the precedent sheet as the lower sheet of the two sheets first abuts the reference wall. After the downstream end in the conveying direction of the precedent sheet shifted in this way abuts the reference wall, the subsequent sheet as the upper sheet slides on the precedent sheet and abuts the reference wall, so that two sheets are completely aligned in the conveying direction. At this time, if the shift amount of two sheets increases, even when the lower precedent sheet first abuts the reference wall, the upper subsequent sheet stops without reaching the reference wall, and hence there is a problem in that an aligning failure of the sheet occurs.
FIG. 10 is a schematic cross-sectional view of a conveying direction aligning portion of the sheet post-processing device. Referring to FIG. 10, a mechanism will be specifically described in which an aligning failure occurs in the sheet post-processing device at the downstream of a sheet overlapping device when there is a variation in the shift amount of two sheets. The sheet overlapping device is disposed at the upstream of the sheet post-processing device, and the subsequent sheet S2 is conveyed from the image forming apparatus body to the sheet post-processing device so that post-processing such as stapling is performed while the subsequent sheet S2 advances with a shift amount in the conveying direction L in relation to the precedent sheet S1.
Subsequently, when a leading end of a conveying direction aligning portion 306 existing at the downstream of an aligning reference wall 317 moves downward from the upper side so as to abut the sheets on an intermediate stacking portion 303, two sheets which are conveyed to the intermediate stacking portion 303 collide with the aligning reference wall 317 so that the sheets are aligned.
Here, as illustrated in FIG. 10A, a case is assumed in which the distance between the tail end of the precedent sheet S1 and the aligning reference wall 317 is denoted by A and the distance between the tail end of the subsequent sheet S2 and the rotation center of the conveying direction aligning portion 306 is denoted by B during or before the aligning operation. In this case, it is desirable to satisfy the relation of A=B with high precision as much as possible so as to suppress the aligning failure.
If a variation in the shift amount of two sheets satisfies the relation of A>B before the aligning operation, the tail end of the subsequent sheet S2 is involved into the nip of the conveying direction aligning portion 306 before the tail end of the precedent sheet S1 reaches the aligning reference wall 317 as illustrated in FIG. 10B during the aligning operation. For this reason, two sheets S1 and S2 are simultaneously nipped in the conveying direction aligning portion 306. In this case, since the conveying direction aligning portion 306 abuts the subsequent sheet S2 from the upper side in a condition in which the friction coefficient between the intermediate stacking portion 303 and the sheet S1 is higher than the friction coefficient between the sheets S1 and S2, only the subsequent sheet S2 is aligned. Accordingly, the tail end of the precedent sheet S1 may not reach the aligning reference wall 317 even when the aligning operation ends, and the sheet bundle takes a posture in which the precedent sheet S1 advances in relation to the subsequent sheet S2 in the conveying direction L, thereby causing the aligning failure of the precedent sheet S1.
Further, if a variation in the shift amount of two sheets satisfies the relation of A<B before the aligning operation, the tail end of the subsequent sheet S2 may not reach the conveying direction aligning portion 306 when the tail end of the precedent sheet S1 reaches the aligning reference wall 317 as illustrated in FIG. 10C during the aligning operation. Accordingly, the tail end of the subsequent sheet S2 may not reach the aligning reference wall 317 even when the aligning operation ends, and the subsequent sheet S2 advances in relation to the precedent sheet S1 in the conveying direction L, thereby causing the aligning failure of the subsequent sheet S2.
The invention is made in view of the above-described circumstances, and it is desirable to provide a sheet conveying apparatus capable of controlling a shift amount of sheets with high precision even when there is a variation in the shift amount between a precedent sheet and a subsequent sheet which are being conveyed.